1. Field of the Invention
The present invention relates to the field of forging metal parts and in particular of complex and warped parts, such as turbine engine blades of large size.
2. Description of the Related Art
For manufacturing metal parts, forging techniques are preferentially applied when they must absorb large stresses in operation. This is the case for compressor or fan blades of turbojets for which the internal stresses are notably generated by the vibrations and centrifugal forces to which they are subject.
Forging consists of plastically deforming a metal bar under the effect of impacts or by applying pressure. Generally, one proceeds stepwise by forming successive blanks which come gradually closer to the finished part. If need be, forging of the part is completed by a calibration phase leading to more accurate shapes.
More specifically, the part is forged by forcing a blank of the latter to be filled by impact or pressure, with an engraved print in a die corresponding to the shape of the part to be obtained. In the case of titanium, as its flow stress strongly depends on temperature, forging is carried out under heat up to a certain limit imposed by the structural change in the material, which modifies its mechanical properties.
The die work operations are generally carried out on mechanical presses with preheated dies. Under these conditions, the forging time is relatively short in order to prevent the part from cooling too fast and the die from heating too much, by thermal conduction between the part and the die itself to the extent that the temperature of the tooling is different from that of the part. Moreover, because of the high level of stresses which it undergoes by contact with the part, a lubricant is deposited on the engraving of the die in order to facilitate flow of the material and to reduce the forging stresses.
The present invention firstly relates to adjusting the tools such as the dies presented above.
The time for making the tools with a conventional method is relatively long as one must proceed with successive touching-up operations.
Indeed, the imprint of the die has not strictly the shape and dimensions of the raw forging part to be obtained. It differs from it by “corrective terms” which compensate the elastoplastic deformations of the tools during the forging. It is not known how to predict these corrective terms accurately, and therefore the die needs to be touched up, subsequently to the measurements performed on the obtained test parts. In so-called precision forging, the oversizes are small, for example 0.8 mm, so that the finished part may be obtained by polishing the raw part with an abrasive belt or, if need be, notably when it is in titanium, by combining chemical machining and polishing with an abrasive belt. For example, this is the case of the blade of the vanes.
An adjustment of a precision forging die is therefore long and costly, as it requires many touch-up operations separated by part forging tests.
When the die is adjusted, i.e., when the obtained forge raw test parts have the sought-after shape and dimensions, this die may be placed in operation for manufacturing series parts. The die gradually deteriorates during operation, and for example, after 1,000-5,000 parts according to the case, it becomes necessary to restore the die or to use another one.
Restoration of a deteriorated die according to a first method, consists of reloading the areas where material has been taken away, and of machining and polishing a new imprint, i.e., rewashing the die by spark machining. According to a second method, the imprint is entirely reformed by machining after removal of the nitride layer (hardened by surface heat or thermomechanical treatments) and removal of a thickness of a few millimeters of material. This technique is designated under the term of rewashing. Restoration of a die or making a new die requires the same adjustments as the initial die. They are therefore also time-consuming and costly.